I have the same sort of problem with my client before but the IP only change several times every ours. It will lead to unstability of the VoIP connection, i.e. will get busy tone or destination not reach problem. I therefore change to a fixed IP service, althougth much expensive, but the problem solved forever!
For the case that IP change every 2 mins, I think you can complaint to your ISP saying that you cannot remotely access you computer since you always do not know your IP.